(Oxo)(Fluoro)–Aluminates in KF–Al₂O₃ System: Thermal Stability and Structural Correlation

Precise investigation of part of the phase diagram of KF–Al₂O₃ system was performed in an experiment combining different techniques. Solidified mixtures of KF–Al₂O₃ were studied by X-ray powder diffraction and high-field solid-state NMR spectroscopy over a wide range of compositions. To help with the interpretation of the NMR spectra of the solidified samples found as complex admixtures, we synthesized the following pure compounds: KAlO₂, K₂Al₂₂O₃₄, α-K₃AlF₆, KAlF₄, and K₂Al₂O₃F₂. These compounds were then characterized using various solid-state NMR techniques, including MQ-MAS and D-HMQC. NMR parameters of the pure compounds were finally determined using first-principles density functional theory calculations. The phase diagram of KF–Al₂O₃ with the alumina content up to 30 mol % was determined by means of thermal analysis. Thermal analysis was also used for the description of the thermal stability of one synthesized compound, K₂Al₂O₃F₂